I've been on a similar quest and ran across a few other tools not mentioned here:
- Stencylworks is a IDE / dev environment for building Flash games very quickly and easily, visual interface for existing modules. it seems to be 100% free, not open source, and key advantages include slick, codeless coding (it's not like Gamemaker's schitzoid script/visual interface). I do worry about the limits of the visual interface - unclear how properly object-oriented it really is; it certainly has simple stuff; presumably javascript script editor's in there somewhere, but haven't seen it yet. It seems to be meant for collaborative development at its core (chat builtin to IDE; all resources and behaviors hosted on a server, user-contributed). That could be GREAT for prototype development. I haven't doen much with it yet but the tutorial was dead simple, and there's no sign of any timeline (which is why normal Flash is so bad for games - timeline metaphor is sooo wrong for games)
I've tried a few other weirder ones too.
Alice is a 3D game / dev environment which is open source - looks kind of clunky.
Kodu, going beyond Gamemaker on the easy/limited spectrum, aimed at kids, lets you build games with an Xbox controller. free from Microsoft Research
Scratch, from MIT Labs, is free and really basic 2D, aimed at getting kids into coding. Visually clunky. Flexible though.
Going waaaay back to Papert's LOGO, Microworlds JR lets you write in logo, allegedly. Needs an update to be useful, IMO.
For me, so far, it's coming down to Construct 2 vs Stencylworks...though I'll check out how easy it is to build a decent 2D game in Unity, as I know and like Unity.